This invention relates to low transmission loss optical waveguides, and more particularly to optical waveguides which have excellent signal transmission properties despite the presence of sharp or multiple lesser bends along the length of the waveguide.
Because of their small size and extremely low signal attenuation characteristics, optical waveguides, which are also referred to as optical fibers for telecommunication applications, are now supplanting coaxial cables and other wide band transmission lines. Optical waveguides are being used not only in telecommunications systems but in high capacity data processing systems, sensor systems and other communications environments as well. A succession of efforts and developments has constantly reduced attenuation losses to levels in the range of 0.16 to 0.28 db/km, with results as low as 0.12 db/km being reported. Optical waveguides for telecommunications are most often constructed to operate in single mode fashion, with the lowest loss wavelength generally being in the range of 1.55 micrometers. This wavelength is in the center of the band in which lowest attenuation is achieved for silica-based fibers. At these. levels of attenuation, extremely long lengths of waveguide can be used between transmitting and receiving units, without intermediate repeaters or amplifiers being employed.
There are, however, certain physical restraints on the ways in which optical waveguides can be used, and an important one of these pertains to the bending which can be tolerated in the waveguide. Signal losses of substantial level can be introduced either because the length of the fiber is turned about a point with a relatively small radius of curvature, or by waviness, called microbending, introduced because of the sheath of the fiber, the way the fiber is wound, or some other mechanical factor. In very general terms, the energy transmitted along an optical waveguide is, as is well known, concentrated about the core and theoretically must maintain wavefront planarity throughout the length of the propagating path. However, viewed in this very general way, it can be seen that a planar wavefront which must propagate through a bend has different path lengths between the center of the core and the outer radius. Since the velocity of light is determined by the media itself, a differential in path length is introduced, and energy losses result because energy is radiated out the side of the waveguide. These losses are dependent upon the extent of the bending that is introduced, i.e., the sharpness of the radius of curvature for pure bending and the number of microbends along the line.
This general description of the effects of bending is much more precisely analyzed in the book entitled "Single-Mode Fiber Optics", 2nd ed., by Luc B. Jeunhomme, Marcel Dekker, Inc., New York & Basel, 1990, pp. 103-115. It is pointed out therein that the difference in phase velocities between plane waves in a core and cladding, which arise at curved path regions, are resisted by the radiation of power away from the guide. Detailed equations are given for qualitative evaluations, there are both bending losses and transition losses, the transition losses following after a bent region when the waveguide returns to a linear path. Jeunhomme gives one equation, on page 106, for the critical radius of curvature R.sub.c at which bending loss "increases from sharply from negligible values to intolerably high values." This is delineated as: EQU R.sub.c =20 .lambda..DELTA.n.sup.-3/2 (2.748-0.996 .lambda./.lambda..sub.c).sup.-3 (Equation 1)
In this equation, .DELTA.n is the difference in index of refraction in percent, between core and cladding, .lambda. is the operating wavelength and .lambda..sub.c is the cutoff wavelength. The cutoff wavelength for a single mode fiber is theoretically predetermined by a geometry and dopant level. In the prior art the approach has been to place the cutoff wavelength below or close to the operating wavelength so as to operate only in single mode and to accept the inherent limitation of bending radius or to increase the difference in index of refraction between core and cladding. This approach reduces the bending loss by further concentrating the field about the core but increases attenuation because of the higher dopant level needed for the higher index differential, higher doping levels having higher attenuation.
There are a substantial number of situations, however, in which it is highly desirable, and even necessary, to have low attenuation loss over a long transmission path which contains a sharp curve or repeatedly deviates from linearity, or is subject to both types of variations. Thus in CATV and computer networks it is often not feasible conveniently to maintain large curvatures and gradual transitions in coupling a source to one or many receivers. A more extreme example is presented by transmission systems in which dynamic changes of curvature may take place. For example, when laying an extremely long length of optical waveguide cable, one would like to be able to monitor transmissions on at least one waveguide during payout. Thus one can determine more precisely when signal amplification might be needed, or identify failures and problems so that corrective action can be taken. Even more stringent requirements are imposed by in a wholly different application, known as tethered vehicle payout systems. In these systems, an optical waveguide is wound concentrically about a longitudinal axis on a bobbin, the turns of the waveguide being lightly adhered to each other. The waveguide is anchored on a stationary receiver and pulled or "peeled" off the bobbin, which is attached to a moving vehicle which moves at least approximately along the longitudinal axis. A video or other signal thus can be fed back from the moving vehicle to the receiver via the waveguide. While the radius of curvature of waveguide wrapped around the bobbin may be relatively small, the radius of curvature is much smaller (of the order of 2 mm or less) at the "peel off" point where the waveguide separates from the bobbin. Given this practical operating requirement, the only resolution of the problem heretofore has been to increase the differential in the indices of refraction between the core and the cladding, and to accept the consequent signal attenuation. Obviously, this has an effect on system performance and reliability that is not desirable.